Fiber lasers and amplifiers are increasingly used in applications that require compact and robust monolithic design, good stability and excellent beam quality. Fiber amplifiers exhibit much higher gain, typically between about 20 decibels (dB) and 40 dB, compared to solid-state amplifiers (typically between about 10 dB and 20 dB). This makes fiber master-oscillator plus power amplifier (fiber-MOPA) systems attractive for amplification of a small signal from a master oscillator to high average and peak powers.
A master oscillator (for example, semiconductor diode) can be easily modulated at high pulse repetition rate, for example, up to about 1 Gigahertz (GHz) while generating pulses with an arbitrary length, for example between about 0.1 nanoseconds (ns) and 10 microseconds (μs). This is one reason why fiber-MOPA systems offer better flexibility and choice in pulse lengths and pulse repetition rates than solid-state lasers.
Most fiber lasers operate at a wavelength in an infrared (IR) wavelength range There is also, however, a growing demand for reliable compact pulsed laser sources in visible and ultraviolet (UV) spectral ranges. This could be satisfied by frequency converting the output of infrared-laser sources.
In general, a narrow linewidth (less than about 0.6 nm), linearly polarization, and high peak power, for example greater than about 1 kilowatt (kW) are required for efficient conversion of IR radiation into visible and UV range. However, conventional high power fiber-laser oscillators usually operate with broader linewidths, for example greater than about 1 nm. Further, high peak power required for efficient harmonic generation is limited by nonlinear effects in fibers such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and Four-Wave Mixing (FWM).
There is a need to overcome the above-discussed deficiencies in linewidth and amplification limitation in fiber-MOPA systems suitable for frequency conversion.